I. Field of the Invention
The present invention relates to communications. More particularly, the present invention relates to a novel and improved method and apparatus for demodulating coherent data in the presence of phase discontinuities.
II. Description of the Related Art
Wireless communication systems including cellular, satellite and point to point communication systems use a wireless link comprised of a modulated radio frequency (RF) signal to transmit data between two systems. The use of a wireless link is desirable for a variety of reasons including increased mobility and reduced infrastructure requirements when compared to wire line communication systems. One drawback of using a wireless link is the limited amount of communication capacity that results from the limited amount of available RF bandwidth. This limited communication capacity is in contrast to wire based communication systems where additional capacity can be added by installing additional wire line connections.
Recognizing the limited nature of RF bandwidth, various signal processing techniques have been developed for increasing the efficiency with which wireless communication systems utilize the available RF bandwidth. One widely accepted example of such a bandwidth efficient signal processing technique is the IS-95 over the air interface standard and its derivatives such as IS95-A and ANSI J-STD-008 (referred to hereafter collectively as the IS-95 standard) promulgated by the Telecommunication Industry Association (TIA) and used primarily within cellular telecommunications systems. The IS-95 standard incorporates code division multiple access (CDMA) signal modulation techniques to conduct multiple communications simultaneously over the same RF bandwidth. When combined with comprehensive power control, conducting multiple communications over the same bandwidth increases the total number of calls and other communications that can be conducted in a wireless communication system by, among other things, increasing the frequency reuse in comparison to other wireless telecommunication technologies. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled xe2x80x9cSPREAD SPECTRUM COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERSxe2x80x9d, and U.S. Pat. No. 5,103,459, entitled xe2x80x9cSYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEMxe2x80x9d, both of which are assigned to the assignee of the present invention and incorporated by reference herein.
FIG. 1 provides a highly simplified illustration of a cellular telephone system configured in accordance with the use of the IS-95 standard. During operation, a set of subscriber units 10a-d conduct wireless communication by establishing one or more RF interfaces with one or more base stations 12a-d using CDMA modulated RF signals. Each RF interface between a base station 12 and a subscriber unit 10 is comprised of a forward link signal transmitted from the base station 12, and a reverse link signal transmitted from the subscriber unit. Using these RF interfaces, a communication with another user is generally conducted by way of mobile telephone switching office (MTSO) 14 and public switch telephone network (PSTN) 16. The links between base stations 12, MTSO 14 and PSTN 16 are usually formed via wire line connections, although the use of additional RF or microwave links is also known.
In accordance with the IS-95 standard each subscriber unit 10 transmits user data via a single-channel, non-coherent, reverse link signal at a maximum data rate of 9.6 or 14.4 kbits/sec depending on which rate set from a set of rate sets is selected. A non-coherent link is one in which phase information is not utilized by the received system. A coherent link is one in which the receiver exploits knowledge of the carrier signals phase during processing. The phase information typically takes the form of a pilot signal, but can also be estimated from the data transmitted.
A coherent reverse link CDMA system is described in co-pending application Ser. No. 08/654,443 entitled xe2x80x9cHIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEMxe2x80x9d, filed May 28, 1996, (hereinafter the ""443 application) assigned to the assignee of the present invention and incorporated by reference herein. In this system, a set of individually gain adjusted subscriber channels are formed via the use of a set of orthogonal subchannel codes having a small number of PN spreading chips per orthogonal waveform period. In a preferred embodiment of this system, pilot data is transmitted via a first one of the transmit channels and power control data transmitted via a second transmit channel. The remaining two transmit channels are used for transmitting non-specified digital data including user data or signaling data, or both. The pilot channel carries a pilot signal which is used to determine phase information which allows demodulation of the data channels.
Before use in demodulation, the pilot signal must be filtered to remove as much transmission-introduced distortion as possible. Typically a low pass filter is used on the pilot signal. Well known in the art of digital signal processing are block filters. Also known are sliding window filters, which give superior performance to a block filters under certain circumstances. One important circumstance for the superior performance of sliding window filters is the lack of phase discontinuity.
Since the pilot signal exists to provide phase information, ideally no phase discontinuity will be present in the signal. However, as a practical matter, the use of cost efficient power amplifiers in the subscriber unit will introduce such phase discontinuities. A typical power amplifier of this type may be a piece-wise linear amplifier, which will produce a discontinuity every time the bias point is switched. Therefore there is a need to design demodulators which are capable of efficiently demodulating in the presence of phase discontinuities.
A novel and improved method and apparatus for coherent demodulation in the presence of phase discontinuities is described. In the exemplary embodiment of this invention, the times when phase discontinuities occur are known apriori by receiver in which demodulation is being performed. In an alternate embodiment, the discontinuity location is signaled to the receiver in advance by the transmitter which generates the signals being demodulated. The pilot signal is prepared for optimal coherent demodulation by the use of two filters: one capable of withstanding the effects of phase discontinuity; a second providing superior filtering performance than the first so long as phase discontinuities are not present. Both filters are simultaneously operated. However, the superior performing filter is selected for use in demodulation whenever possible. In the exemplary embodiment of the present invention, a sliding window filter is employed for the superior performing filter absent phase discontinuity and a block filter is employed for use when phase discontinuities are present. Only when the receiver detects that the sliding window filter will integrate over a phase discontinuity will the receiver select the output of the block filter rather than the output of the sliding window filter for use as the pilot signal employed in coherent demodulation.
This technique can be used whenever it can be known by the receiver the times when phase discontinuities are likely to occur. They may occur periodically and so the receiver can calculate when they will arrive. Alternatively a transmitter can signal when a phase discontinuity will occur. Similarly, if a transmitter signals following a phase discontinuity that such an event occurred, a receiver can compensate for this effect by delaying the demodulation by such time as necessary to account for the arriving phase discontinuity information.